feat(02-06): implement PermissionsViewModel with multi-site scan and SitePickerDialog

- PermissionsViewModel extends FeatureViewModelBase, implements RunOperationAsync
- Multi-site mode: loops SelectedSites; single-site mode: uses SiteUrl
- ExportCsvCommand and ExportHtmlCommand enabled only when Results.Count > 0
- OpenSitePickerCommand uses dialog factory pattern (Func<Window>?)
- OnTenantSwitched clears Results, SiteUrl, SelectedSites
- Flat ObservableProperty booleans (IncludeInherited, ScanFolders, etc.) build ScanOptions record
- SitePickerDialog XAML: filterable list with CheckBox column, Title, URL columns
- SitePickerDialog code-behind: loads sites on Window.Loaded, exposes SelectedUrls
- ISessionManager interface extracted for testability (SessionManager implements it)
- StartScanAsync_WithMultipleSiteUrls_CallsServiceOncePerUrl test passes (60/60 + 3 skip)

SharepointToolbox/Views/Dialogs/SitePickerDialog.xaml

@@ -0,0 +1,60 @@
+<Window x:Class="SharepointToolbox.Views.Dialogs.SitePickerDialog"
+        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
+        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
+        Title="Select Sites" Width="600" Height="500"
+        WindowStartupLocation="CenterOwner" ShowInTaskbar="False"
+        Loaded="Window_Loaded">
+  <Grid Margin="12">
+    <Grid.RowDefinitions>
+      <RowDefinition Height="Auto" />
+      <RowDefinition Height="*" />
+      <RowDefinition Height="Auto" />
+      <RowDefinition Height="Auto" />
+    </Grid.RowDefinitions>
+
+    <!-- Filter row -->
+    <DockPanel Grid.Row="0" Margin="0,0,0,8">
+      <TextBlock Text="Filter:" VerticalAlignment="Center" Margin="0,0,8,0" />
+      <TextBox x:Name="FilterBox" TextChanged="FilterBox_TextChanged" />
+    </DockPanel>
+
+    <!-- Site list with checkboxes -->
+    <ListView x:Name="SiteList" Grid.Row="1" Margin="0,0,0,8"
+              SelectionMode="Single"
+              BorderThickness="1" BorderBrush="#CCCCCC">
+      <ListView.View>
+        <GridView>
+          <GridViewColumn Header="" Width="32">
+            <GridViewColumn.CellTemplate>
+              <DataTemplate>
+                <CheckBox IsChecked="{Binding IsSelected, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}"
+                          HorizontalAlignment="Center" />
+              </DataTemplate>
+            </GridViewColumn.CellTemplate>
+          </GridViewColumn>
+          <GridViewColumn Header="Title" Width="200" DisplayMemberBinding="{Binding Title}" />
+          <GridViewColumn Header="URL" Width="320" DisplayMemberBinding="{Binding Url}" />
+        </GridView>
+      </ListView.View>
+    </ListView>
+
+    <!-- Status text -->
+    <TextBlock x:Name="StatusText" Grid.Row="2" Margin="0,0,0,8"
+               Foreground="#555555" FontSize="11" />
+
+    <!-- Button row -->
+    <DockPanel Grid.Row="3">
+      <Button x:Name="LoadButton" Content="Load Sites" Width="80" Margin="0,0,8,0"
+              Click="LoadButton_Click" />
+      <Button Content="Select All" Width="80" Margin="0,0,8,0"
+              Click="SelectAll_Click" />
+      <Button Content="Deselect All" Width="80" Margin="0,0,8,0"
+              Click="DeselectAll_Click" />
+      <StackPanel Orientation="Horizontal" HorizontalAlignment="Right" DockPanel.Dock="Right">
+        <Button Content="OK" Width="70" Margin="4,0" IsDefault="True"
+                Click="OK_Click" />
+        <Button Content="Cancel" Width="70" Margin="4,0" IsCancel="True" />
+      </StackPanel>
+    </DockPanel>
+  </Grid>
+</Window>